kconfig: qconf: clean deprecated warnings

The recommended way to initialize a null string is with
QString(). This is there at least since Qt5.5, with is
when qconf was ported to Qt5.

Fix those warnings:

	scripts/kconfig/qconf.cc: In member function ‘void ConfigItem::updateMenu()’:
	scripts/kconfig/qconf.cc:158:31: warning: ‘QString::null’ is deprecated: use QString() [-Wdeprecated-declarations]
	  158 |    setText(noColIdx, QString::null);
	      |                               ^~~~
	In file included from /usr/include/qt5/QtCore/qobject.h:47,
	                 from /usr/include/qt5/QtWidgets/qwidget.h:45,
	                 from /usr/include/qt5/QtWidgets/qmainwindow.h:44,
	                 from /usr/include/qt5/QtWidgets/QMainWindow:1,
	                 from scripts/kconfig/qconf.cc:9:

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>